The present invention relates to a method of producing mould parts on a mould string apparatus and to a string moulding apparatus for producing mould parts, in which the mould parts are produced by introducing a compressible particulate moulding material in the moulding chamber and then moving a squeeze plate and a pivoted squeeze plate towards each other to squeeze the mould part.
A method and apparatus of this general kind is known from U.S. Pat No. 5,647,424. According to this method, an apparatus comprising a moulding chamber between a squeeze plate and a pivoted squeeze plate carries out a number of sequential movements in order to produce a mould part The moulding process comprises the steps of:
charging the moulding chamber with compressible mould material, e.g. clay-bonded green sand,
bilateral pressing the mould material between a squeeze plate and a pivoted squeeze plate thus forming a mould part,
retracting the pivoted squeeze plate and pivoting the pivoted squeeze plate out of the way,
moving the squeeze plate towards and past the pivoted squeeze plate for pushing the mould out from the moulding chamber and bringing it into abutment with a mould having been produced immediately before, and
moving the squeeze plates back to their respective starting positions, after which a new cycle begins.
The squeezing of the mould process according to U.S. Pat. No. 5,647,424 is bilateral, i.e. both the squeeze plate and the pivoted squeeze plate move into the moulding chamber during the squeezing of the mould. The advantage of bilateral squeezing is the in the degree of compaction of the sand and the squeeze plate and the pivoted squeeze plate is equal, hence the degree of hardness of the mould surfaces produced at these plates is equal. However, often the squeezed mould part will not be placed at the moulding chamber front at the end of the squeezing process. This has the disadvantage that a vacuum will be drawn when the pivoted squeeze plate is stripped from the mould part and retracted from the moulding chamber. The vacuum can damage the mould part or reduce the quality of the mould part by tearing off pieces of the mould part and by sucking in sand which deposits on the surface of the mould part.
This problem has up to now been solved by moving the pivoted squeeze plate so slowly out of the moulding chamber that the vacuum is reduced by air flowing in through nozzles and openings between the pivoted squeeze plate an the moulding chamber. Another solution has been to move the squeeze plate and the pivoted squeeze plate simultaneously and with the same speed towards the front of the moulding chamber after the squeezing process so that the mould part is transported to the chamber front. Both solutions have the disadvantage that the cycle time is significantly increased.
It is the object of the invention to provide a method of producing mould parts on a mould string apparatus of the kind referred to above, in which the bilateral squeezing process can be controlled in a better way. By controlling the velocity of the squeeze plate and the velocity of the pivoted squeeze plate independently, the squeezing process can be controlled such that the mould part can be positioned at the moulding chamber front at the end of the squeezing process.
The velocity of the squeeze plate and the pivoted squeeze plate may be controlled such that they move in the same direction during at least a part of the squeezing of the mould. The velocity of the squeeze plate and the pivoted squeeze plate may also be controlled such that either the squeeze plate or the pivoted squeeze plate is slowed down abruptly for creating a shock effect The velocity of the squeeze plate and the pivoted squeeze plate may also be controlled such that the pivoted squeeze plate is reversed during the squeezing operation. The velocity of the squeeze plate and the pivoted squeeze plate may also be controlled such that they move towards one another with different velocity during at least a part of the squeezing of the mould. The velocity of the squeeze plate and the velocity of the pivoted squeeze plate may be controlled according to a predetermined velocity versus time profile. The velocity of the pivoted squeeze plate is controlled such that the pivoted squeeze plate is positioned at the moulding chamber front at the end of the squeezing of the mould.
It is another object of the invention to provide a string moulding apparatus for producing mould parts of the kind referred to above, in which the bilateral squeezing process can be controlled in a better way.
By controlling the velocity of the squeeze plate and the velocity of the pivoted squeeze plate independently, the squeezing process can be controlled such that the mould part will be placed at the moulding chamber front at the end of the squeezing process.
According to an embodiment of the invention, the actuator driving the squeeze plate and the actuator driving the pivoted squeeze plate are independently powered. According to another embodiment of the invention a first hydraulic actuator driving the squeeze plate is powered by a first pump and a second hydraulic actuator driving pivoted squeeze plate is powered by a second pump. The apparatus may comprise a sensor for producing a signal corresponding to the velocity of the squeeze plate and comprising a sensor for producing a signal corresponding to the velocity of the pivoted squeeze plate. The apparatus may advantageously comprise a controller that receives the signals from the sensors and controls the velocity of the squeeze plate and the pivoted squeeze plate in response to these signals. In order to allow flexible operation of the apparatus, for example when shifting to another type of mould part, a number of operator selectable or automatically selectable predetermined velocity versus time profiles for the squeeze plate and the pivoted squeeze plate are stored in the controller. The controller may control the velocity of the squeeze plates in a closed loop manner for example according to a PID control function.